This invention relates to a method and apparatus for object surface determination and, in particular, a method and apparatus for object surface determination which utilizes projected electromagnetic radiation.
In systems in use today for determining the positions of points on an object surface, a projector/detector sensor is used to develop data which is then analyzed and processed to obtain the surface point locations. In a typical system, the sensor comprises a projector which projects electromagnetic radation such as, for example, optical radiation, at the object surface. The projected electromagnetic radiation confronts or intersects the surface and gives rise to reflected radiation. The reflected radiation is then collected by a lens which focuses the reflected radiation onto a radiation detector, the lens and detector forming parts of the sensor. An analyzing and processing unit is fed the output of the sensor, i.e., the sensor detector output, and using conventional triangulation techniques determines the locations of the points on the object surface.
In order for the aforesaid system to provide accurate surface point determinations, the power of the reflected radiation collected by the lens and coupled to the radiation detector must fall well within the useable dynamic range of the sensor detector. By useable dynamic range is meant the range between the power level at which the detector is saturated (i.e., provides no increase in output for an increase in input power or when deleterious effects are introduced) and the power level corresponding to a level significantly above the noise of the system. Where the object surfaces being analyzed may have diverse reflective characteristics from surface to surface such as, for example, those found in solder joints, it is not always possible to achieve the aforementioned condition. Thus, with these types of surfaces, the dynamic range of the reflected radiation intensity and, therefore, reflected power, for a constant on time of the projector, may be 1000:1, while the useable dynamic range of the radiation sensor detector may be only about 6:1. As a result of this, the system performance suffers and in many cases is unsatisfactory.
One way of attempting to avoid this problem is to project radiation at a number of different power levels at the surface and obtain data from the sensor detector for each projection. The data from the different projections which is considered most acceptable is then selected for further processing. This procedure, however, is time consuming and increases the overall time for surface examination by N times, where N is the number of projections required to obtain acceptable data for worst case conditions. A typical range for N would be from 6 to 15, thereby significantly increasing the overall time of operation.
It is therefore an object of the present invention to provide a system of the above type wherein the dynamic range of the reflected radiation intensity of the system is not limited to the dynamic range of the sensor detector of the system.
It is a further object of the present invention to provide a system of the above type wherein a relatively large diversity of reflective characteristics of object surfaces can be accommodated, while not significantly increasing the operating time of the system.